The present invention relates generally to orthopaedic implant devices of the type having a porous surface into which bone tissue can grow or surgical cement can enter and, more particularly, to a method of bonding a porous surface structure of titanium or a titanium alloy onto a substrate of a cobalt-based alloy, whereby enhanced bonding strength and corrosion resistance is achieved.
Orthopaedic implant devices commonly include a porous surface to enhance attachment of the device to adjacent bone tissue. Consequently, various methods have developed for manufacturing an implant device having a porous surface. One such method involves securing a porous fiber metal structure, i.e., a wire mesh pad, to a portion of the surface of the device where bone ingrowth or attachment is desired. While the fiber metal pad generally provides a suitable porous surface, efforts to develop improved methods of attaching the pad to the implant surface continue. For instance, it is a desired to further improve the bonding strength and corrosion resistance of the attachment interface.
In several orthopaedic implant applications, it is desired to combine dissimilar metals in order to take advantage of the particular strength, biocompatibility, and corrosion resistance properties of the respective metals. For instance, as disclosed in UK Patent GB 2142544 B to Medcraft, it is known to combine a cobalt-based alloy with titanium or a titanium alloy, wherein one constitutes the substrate of an implant device and the other constitutes a wire mesh that is diffusion bonded to the substrate to provide a porous surface. However, diffusion bonding of commercially pure titanium porous structure (such as a porous pad) onto cobalt-based alloys requires elevated temperatures which reduces corrosion resistance of the composite structure. Furthermore, diffusion bonding of titanium porous structure directly onto a cobalt-based alloy does not produce necessary bond strength in the structure.
In another application involving a cobalt-based alloy and titanium, the femoral component of a knee prosthesis is fabricated by casting a cobalt-based alloy substrate and then securing a titanium fiber metal pad to the surface of the substrate by first plasma spraying a titanium coating on the surface. This bonding method is disclosed in U.S. Pat. No. 4,969,907 to Koch et al.
Both of the aforementioned bonding arrangements result in the formation of an alloy at the interface between components that tends to exhibit some corrosion at the interfacing layers. Nevertheless, it desired to provide an improved method of bonding a titanium porous surface to the surface of a cobalt-based alloy substrate, wherein bonding strength and corrosion resistance are enhanced.